The hypothesis of this study is that alterations in the hypervariable region of the EIAV LTR enhancer are a major determinant of EIAV virulence, being directly responsible for regulation of EIAV replication in equine macrophages. The overall goal is to understand how enhancer variability regulates macrophage replication in acute or relapsing viremic infection and chronic asymptomatic "latent" infection/viremic suppression. Specifically, the PI wishes to establish whether hypervariability in the LTR occurs to decrease macrophage-specific viral expression and facilitate progression from symptomatic to asymptomatic EIAV infection. To this end, the PI seeks (1) to identify LTR enhancer sequences required for viral expression in macrophages, primary fibroblasts and established fibroblastoid cell lines using electrophoretic gel shift assays (EMSAs) and transient expression assays; (2) to determine if enhancer Date motifs thus identified regulate EIAV replication in the context of an infectious molecular clone; (3) to determine if hypervariability in the EIAV LTR enhancer controls cell tropism; and (4) to determine if hypervariability in the EIAV LTR enhancer determines virus persistence or virulence in horses in vivo.